Static mixer assemblies are known in the art. Many approaches have been used in the design of static mixer assemblies depending upon the desired degree of mixing required and the pressure drop or loss which may be tolerated across the static mixing arrangement for a required volumetric flow rate through the static mixer assembly. Generally, the greater the volumetric flow rate through a static mixer assembly, the greater will be the pressure drop required across the assembly in order to achieve the desired flow rate.
A high degree of mixing is desirable in combination with a low pressure drop across the static mixer assembly and a high volumetric flow rate. Typically, dimensionless parameters such as System loss coefficient or Darcy friction factor are used to assess the head-loss or pressure drop for any particular volumetric flow rate. It is desirable that these values be as low as possible whilst the static mixer assembly provides homogeneous mixing. By homogeneous mixing it is meant, for instance, that the concentration of one or more ingredients introduced at one or more locations upstream or downstream of the static mixer assembly is uniformly distributed both axially and radially at a location downstream of the static mixer assembly. So, for instance, if the static mixer achieves homogeneous mixing at a distance L downstream of the static mixer assembly, then any element of the cross sectional area at L will exhibit substantially the same concentration of the ingredient, say within +/−1% of the mean concentration of the ingredient, and this will also be the case for any cross section downstream of L. In the industry, time averaged values for coefficient of variation of ingredient concentration are used as a measure of mixing homogeneity. A value of 0.05 or less is considered to indicate good mixing. Details of the measurement of this parameter are set out hereinafter.
It is also desirable for simple construction materials and techniques to be utilised in the manufacture of the static mixer assembly whereby manufacturing costs can be kept low. Ideally, the mixing required would be achieved with few, simple, mixing elements making up the static mixer assembly within a pipe through which the fluid flows. By definition, the mixing elements are rigidly fixed within the pipe (or within an insert to be fitted into a pipe), this being a fundamental feature defining a static mixer assembly (i.e. no moving mechanical parts are required, mixing is induced by the flow of a fluid through the static mixer assembly).
The mixing elements of prior art static mixer assemblies are typically obstacles around which the fluid, flowing along a pipe, is constrained to flow. As fluid passes around the obstacles, vortices may be initiated at the edges of the obstacles and these will detach from the obstacles at regular time intervals. As the vortices proceed along the pipe downstream of the obstacles, additive to be mixed into the fluid may be taken up in the vortices and so redistributed throughout the fluid by the vortex flow. However, the periodicity of detachment of vortices may lead to axial inhomogeneity in the distribution of the mixed-in additive. A static mixer assembly is disclosed in U.S. Pat. No. 5,839,828, which discloses a static mixer arrangement comprising a circumferential flange extending inwards from an internal wall of a pipe, the flange having at least a pair of opposed flaps extending inwards therefrom and inclined in the direction of fluid flow.
U.S. Pat. No. 7,316,503 discloses a static mixer for low viscosity fluid containing inbuilt devices arranged in a pipe conducting the fluid. A plurality of flow obstacles are disposed to define constrictions therebetween for flow of a viscous fluid therethrough and to impart a flow of a first order in the flow of viscous fluid passing through the constrictions, including vortex spheres which periodically separate off the obstacles, said to produce radial and axial inhomogeneities in the form of axial concentration differences in the flow of viscous fluid.
Each primary flow obstacle of this prior art has a geometrically modified area for at least one surface and an edge thereof to induce local flows of a second order in the flow of viscous fluid passing thereover whereby the local flow of second order is superimposed on the flow of first order to compensate for radial and axial inhomogeneities in the viscous fluid produced by the flow of first order. The flow obstacles disclosed in U.S. Pat. No. 7,316,503 are complex in shape and may require multiple manufacturing steps for their formation.
Despite the existence of various known static mixer arrangements, there is a developing need for mixers of this type with better mixing efficiency and homogeneity of mixed fluid in combination with reduced static mixer assembly length, to facilitate the incorporation of the static mixer assembly into a plant, and with reduced pressure drop across the static mixer assembly for a desired volumetric flow rate, to reduce flow resistance and pumping requirements for the plant into which the static mixer is to be incorporated.